--- a/layout/style/FontFaceSet.cpp
+++ b/layout/style/FontFaceSet.cpp
@@ -102,16 +102,17 @@ FontFaceSet::FontFaceSet(nsPIDOMWindowIn
   , mDocument(aDocument)
   , mResolveLazilyCreatedReadyPromise(false)
   , mStatus(FontFaceSetLoadStatus::Loaded)
   , mNonRuleFacesDirty(false)
   , mHasLoadingFontFaces(false)
   , mHasLoadingFontFacesIsDirty(false)
   , mDelayedLoadCheck(false)
   , mBypassCache(false)
+  , mPrivateBrowsing(false)
 {
   MOZ_ASSERT(mDocument, "We should get a valid document from the caller!");
 
   nsCOMPtr<nsIGlobalObject> global = do_QueryInterface(aWindow);
 
   // If the pref is not set, don't create the Promise (which the page wouldn't
   // be able to get to anyway) as it causes the window.FontFaceSet constructor
   // to be created.
@@ -132,16 +133,21 @@ FontFaceSet::FontFaceSet(nsPIDOMWindowIn
     if ((NS_SUCCEEDED(docShell->GetLoadType(&loadType)) &&
          ((loadType >> 16) & nsIWebNavigation::LOAD_FLAGS_BYPASS_CACHE)) ||
         (NS_SUCCEEDED(docShell->GetDefaultLoadFlags(&flags)) &&
          (flags & nsIRequest::LOAD_BYPASS_CACHE))) {
       mBypassCache = true;
     }
   }
 
+  // Same for the "private browsing" flag.
+  if (nsCOMPtr<nsILoadContext> loadContext = mDocument->GetLoadContext()) {
+    mPrivateBrowsing = loadContext->UsePrivateBrowsing();
+  }
+
   if (!mDocument->DidFireDOMContentLoaded()) {
     mDocument->AddSystemEventListener(NS_LITERAL_STRING("DOMContentLoaded"),
                                       this, false, false);
   }
 
   mDocument->CSSLoader()->AddObserver(this);
 
   mUserFontSet = new UserFontSet(this);
@@ -1444,23 +1450,16 @@ FontFaceSet::SyncLoadFontData(gfxUserFon
     aBuffer = nullptr;
     aBufferLength = 0;
     return rv;
   }
 
   return NS_OK;
 }
 
-bool
-FontFaceSet::GetPrivateBrowsing()
-{
-  nsCOMPtr<nsILoadContext> loadContext = mDocument->GetLoadContext();
-  return loadContext && loadContext->UsePrivateBrowsing();
-}
-
 void
 FontFaceSet::OnFontFaceStatusChanged(FontFace* aFontFace)
 {
   AssertIsMainThreadOrServoFontMetricsLocked();
 
   MOZ_ASSERT(HasAvailableFontFace(aFontFace));
 
   mHasLoadingFontFacesIsDirty = true;
@@ -1859,17 +1858,17 @@ FontFaceSet::UserFontSet::SyncLoadFontDa
   }
   return mFontFaceSet->SyncLoadFontData(aFontToLoad, aFontFaceSrc,
                                         aBuffer, aBufferLength);
 }
 
 /* virtual */ bool
 FontFaceSet::UserFontSet::GetPrivateBrowsing()
 {
-  return mFontFaceSet && mFontFaceSet->GetPrivateBrowsing();
+  return mFontFaceSet && mFontFaceSet->mPrivateBrowsing;
 }
 
 /* virtual */ void
 FontFaceSet::UserFontSet::DoRebuildUserFontSet()
 {
   if (!mFontFaceSet) {
     return;
   }

--- a/layout/style/FontFaceSet.h
+++ b/layout/style/FontFaceSet.h
@@ -260,17 +260,16 @@ private:
   nsCSSFontFaceRule* FindRuleForUserFontEntry(gfxUserFontEntry* aUserFontEntry);
 
   nsresult StartLoad(gfxUserFontEntry* aUserFontEntry,
                      const gfxFontFaceSrc* aFontFaceSrc);
   nsresult CheckFontLoad(const gfxFontFaceSrc* aFontFaceSrc,
                          nsIPrincipal** aPrincipal,
                          bool* aBypassCache);
   bool IsFontLoadAllowed(nsIURI* aFontLocation, nsIPrincipal* aPrincipal);
-  bool GetPrivateBrowsing();
   nsresult SyncLoadFontData(gfxUserFontEntry* aFontToLoad,
                             const gfxFontFaceSrc* aFontFaceSrc,
                             uint8_t*& aBuffer,
                             uint32_t& aBufferLength);
   nsresult LogMessage(gfxUserFontEntry* aUserFontEntry,
                       const char* aMessage,
                       uint32_t aFlags,
                       nsresult aStatus);
@@ -354,14 +353,18 @@ private:
 
   // Whether CheckLoadingFinished calls should be ignored.  See comment in
   // OnFontFaceStatusChanged.
   bool mDelayedLoadCheck;
 
   // Whether the docshell for our document indicated that loads should
   // bypass the cache.
   bool mBypassCache;
+
+  // Whether the docshell for our document indicates that we are in private
+  // browsing mode.
+  bool mPrivateBrowsing;
 };
 
 } // namespace dom
 } // namespace mozilla
 
 #endif // !defined(mozilla_dom_FontFaceSet_h)